Bracken fern (BF) is a plant used as a food delicacy and salad green in the United States, Canada, and Japan. The consumption of BF predisposes to an augmented risk for human eosophageal carcinoma. BF is a forage contaminant responsible for a high incidence of naturally occurring bladder cancer in cattle, and intestinal and bladder cancer in sheep. Administration of BF to experimental animals produces a wide variety and high incidence of bladder and intestinal cancer. Carcinogenic activity of BF can be inhibited by various chemicals. BF carcinogen is excreted in milk of cows fed BF. Such milk produces intestinal and bladder cancer in rats by feeding. Humans ingesting such milk might be unwittingly exposed to a naturally occurring plant carcinogen. BF carcinogen is present in tannin-free extracts of the plant. Such extracts produce a high incidence of cancer in rats by oral administration. Carcinogen present in tannin-free and tumor producing BF extracts will be identified and tested for carcinogenic activity utilizing animal and microbiological systems. Isolation studies of BF carcinogen from milk and urine will be conducted. Bacterial mutagenesis and intestinal and bladder ornithine decarboxylase induction will be used as principal monitors of isolation. Carcinogenesis of bioactive fractions will be studied. The role of L-tryptophan or tannin as promoters of BF carcinogenesis will be investigated.